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1200A Feeder Size

1200A Feeder Size

1200A Feeder Size

(OP)
Per NEC 240.4(B), a 277/480V, 800A feeder is allowed to be the industry standard of 2 sets of 3-1/2”C, (4)#500, (1)#2G even though the ampacity of the feeders are only 380A.  However, it seems that I’ve seen this standard applied to 1200A and 1600A feeders as well, even though 240.4(C) requires any OC device over 800A to have conducters rated equal to or greater than the device rating.  

So am I correct in assuming that the proper size for a 1200A feeder would be 4 sets of 3”C, (4)#350, (1)#3/0G?  Or is there some loophole I’m not aware of that would allow three sets of #500's to be applied to this circuit as well?

RE: 1200A Feeder Size

No loophole.  If installed per code a 1200A circuit requires more conductor than just 3 sets of 500.  Your 4 sets of 350 works if you do not need to count the neutral as a current carrying conductor.  If something, such as triplin harmonics, causes you to consider the neutral as a current carrying conductor, you will need 4 sets of 500 for that 1200A circuit to account for the derating.

RE: 1200A Feeder Size

For 1200A feeders we often use 3 sets each: 4#600, 1#3/0 - 4".  Have not had any complaints about pulling 600MCM copper.

RE: 1200A Feeder Size

(OP)
In the consulting engineering field, we've always been told not to go over #500 due to skin effect.  Any comments on that?  I'd be interested in hearing what other's opinions are.  

RE: 1200A Feeder Size

Comment: The 500MCM conductor ampacity of 380A is related to RHW, THHW, THW, THWN, XHHW, USE, and ZW Type 75°C(167°F).
 insulated conductors.
75°C temperature rating is maximum required for landing the conductors in circuit breaker terminals.
If there are switches and fuses, 90°C conductor temperature rating may be considered, if the Utility permits.
90°C insulated conductor types are RHH, THHN, THHW, XHHW, XHHW-2, USE-2, THWN-2, etc. as they appear in NFPA 70-2002 NEC Table 310.16 on page 70-144. The allowable ampacity for 500MCM for these conductors is 430A.

RE: 1200A Feeder Size

The rule of thumb about not exceeding 500kcmil is based on cost effectiveness of the larger conductors, ability to pull in conduit and stress on terminals. I've used 600 and rarely 750 in special situations. Some utilities will use up to 1000. Some utilities dont permit >500 (CU) or >750 (AL) for termination on transformer bushings due excessive stress and tendency for the bushings to fail.

You can take advantage of the higher temperature insulations even if your terminations don't allow, if deratings are necessary for >3 conductors in a conduit or in underground duct banks.

RE: 1200A Feeder Size

Up to 800A, the NEC allows use of "next larger standard size" overcurrent device for feeder conductor protection.  Above 800A, this exception does NOT apply.

So a feeder protected by a 1200A breaker, for example, must have an ampacity of at least 1200A - 1199A is not acceptable.  I'd suggest 4-350 kcmil copper per phase or 3-600 kcmil copper per phase for a 1200A feeder.

As others have mentioned, the upper limit of 500 kcmil is really an economic one.  Beyond 500 kcmil, the added conductor cross-section does not provide a corresponding increase in cable ampacity, for a variety of reasons.  Installation of larger copper conductors is also more difficult due to weight and stiffness of the cable.  

RE: 1200A Feeder Size

Suggestion: Current limiting fuses may be applied upstream of the breaker to reduce short circuit current levels and increase the conductor temperature from 75°C to 90°C. This type of application will allow to apply conductors with higher ampacities, e.g. 500MCM Cu ampacity will increase from 380A to 430 amps, which is not a bad deal.

RE: 1200A Feeder Size

What do short circuit current levels have to do with which conductor temperature can be used for conductor ampacity?  Lugs are rated 75 or 90 degrees C and there is nothing that can be done about that through fault current limitation.

RE: 1200A Feeder Size

jbartos, lug ratings don't change by simply installing fuses. What is your basis for this statement?

RE: 1200A Feeder Size

Comment on the previous posting: 75°C temperature rated conductor limit is normally aligned with molded case circuit breakers. To circumvent this restriction, the 90°C temperature rated conductors may be terminated on fuse blocks that accept that temperature. Alternately, a terminal box will do to transition 90°C rated insulated conductors to 75°C insulated rated conductors.
If fuses are designed installed, why not to design and install the current limiting ones? They will reduce the short circuit level downstream within the power distribution system, which is always considered a safety measure as well as it prolongs contact life-cycles.

RE: 1200A Feeder Size

Isn't there still some type of cable lug needed to make the connection? I haven't seen too many 90°C rated lugs in production, though I understand that some are available somewhere.

RE: 1200A Feeder Size

Check UL White Book for ratings

RE: 1200A Feeder Size

Comment: How would all those 90°C conductors be terminated at equipment that permits 90°C, e.g. fuse blocks, terminal blocks, disconnect switches, etc.????

RE: 1200A Feeder Size

If Overcurrent devise is 100 amp or greater use 75 degree column for the ampacity you apply derating factors ( fill and temperature ) against the 90 degree column.

RE: 1200A Feeder Size

This is the basis of NEC Art 110.3(B) 'Listed and Labeled'. Most lugs are listed at no more than 75°C, which means that the conductor must be derated to the 75°C column in Art 310.

RE: 1200A Feeder Size

Any lug, whether mechanical or compression, will have a temperature rating.

RE: 1200A Feeder Size

Just checked Cutler-Hammer and SquareD safety switches. Lugs on fused and non-fused switches are mechanical type, rated 60C for sizes up to 100A and 75C above 100A.

RE: 1200A Feeder Size

Comment on the previous posting: The higher temperature rated conductor is better for fused disconnect switches since fuses can become very hot thus damaging the conductor insulation. I noticed that the Square D current Digest Catalog 173 list 75°C conductor temperature rating for circuit breakers in enclosures. They resemble switches. I do not see similar temperature limitation for fusible disconnect switches and lugs in Digest 173.

RE: 1200A Feeder Size

Although the actual terminal may carry a 90 deg rating from the terminal manufacturer, when it is installed in a piece of equipment, the equipment as a whole generally is 75 degrees (larger than 100A), because the manufacturers have not been testing equipment at the higher levels.

RE: 1200A Feeder Size


Code stranding on 500kcmil is 37 strand—where 600kcmil is 61 strand, making it a little easier terminate.    [99NEC Ch9 table 5A]
  

RE: 1200A Feeder Size

The temperature rating applys to current montoring devises, OL Relay Blocks, Fuses,and Circuit Breakers (not 100% rated.
This derating only applies to equipment rated less than 600 Volts. Bolted Pressure Contact Switches and 100% Rated Circuit Breakers can be terminated a 90 degree if so labeled. Running a conductor at 90 degree rating is incresing circuit losses and voltage drop

RE: 1200A Feeder Size

Many 100% rated devices require the use of 90 degree conductors AT the 75 degree ampacity.

RE: 1200A Feeder Size

Comment on the previous alehman posting: I agree that all devices that have temperature rating limit on the conductor temperature termination have to have these limits satisfied. Visit
http://www.nmsu.edu/~tdi/pdf-resources/cc47.pdf
for related examples

RE: 1200A Feeder Size

I think the previous posting sums it up.

Terminals are rated based on the mutual heating effects. If rated for 75C then the conductor terminated must be sized based on its 75C rating. This is because of the mutual effect of heat from the conductor and from the device. The concern is for effects of heat on the device, the connection itself and the insulation on the conductor. Using a compression lug vs. mechanical doesn't typically change the rating.

RE: 1200A Feeder Size

Comment on alehman (Electrical) Mar 27, 2004 marked ///\\\
jbartos,
See note below tables on page 31 here-
http://ecatalog.squared.com/techlib/displaydocument.cfm?id=3100CT9801&action=view
///Yes, these are safety switches in Section 3 Class 3110 with advertised lug temperature ratings 60°C and 75°C. However, I was referring to disconnect switches Section 7 Class 9422, in my posting on Mar 27.\\\

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